1. Field of the Invention
The present invention relates to a raised and recessed sheet material, and a vehicle panel and a laminated structure using the same, and particularly to a sheet material having extremely improved stiffness due to the formation of raised and recessed portions, and a laminated structure and a vehicle panel having a high degree of stiffness and excellent energy absorbing properties obtained by using such a sheet material.
2. Description of Related Art
Conventionally, in order to reduce the weight of a vehicle, it is considered to replace a material of a vehicle component made of a steel sheet or the like with a material having a light weight, such as an aluminum alloy sheet, and vehicle components made of such light weight material are practically used. However, it goes without saying that the stiffness required for the component needs to be maintained even when the weight of the component is reduced.
In order to improve the stiffness of a sheet material without increasing its thickness, it has been considered to improve the stiffness by forming the sheet material so as to have a corrugated shape or a raised and recessed pattern. For example, Patent Document 1 proposes, as a material of a vehicle component called a heat insulator, a sheet material having many raised portions formed by embossing in order to secure an isotropic stiffness of the sheet material without increasing its thickness. Patent Documents 2-6, for example, propose various sheet materials which are used for various applications as well as the heat insulator and which have improved stiffness owing to formation of raised and recessed portions by embossing, for example.
In fact, the sheet material having many raised and recessed portions, such as the one of Patent Document 1, has a stiffness about two times that of a sheet material having no raised and recessed portions, making it possible to reduce its weight by about 20%. The sheet material having the corrugated shape have different degrees of stiffness in respective different directions, so that even if its stiffness is improved in one direction, a desired improvement of stiffness may not be achieved in the other directions. Thus, it has not been clear as to the optimal configuration of the raised and recessed portions which can improve the stiffness of the sheet material without increasing its thickness. There is always a demand for improving the stiffness of the sheet material at higher levels than ever. Further, in various machines and apparatus as well as automobiles, there is a demand for minimizing the weight of the part made of the sheet material. Moreover, there is not only the demand for the reduction of the weight, but also a need for reduction of the material cost. Also, the sheet material (planar material) is required to have improved stiffness regardless of its material.
Further, there is a demand for improving the stiffness of a laminated structure or a vehicle panel, as compared with conventional ones, by using the above-described raised and recessed sheet material having a high effect of improvement of the stiffness as a component of the laminated structure or an inner or outer panel of the vehicle.
Therefore, the present inventor disclosed in Non-Patent Document 1, that where the sheet material is obtained by repeatedly forming H-shaped patterns on a flat sheet so as to form a shell structure with raised and recessed portions, the sheet material can have a bending stiffness more than ten times that of the flat sheet in any direction, making it possible to improve the weight reduction percent value to 50% or more. However, the present inventor made a further study on the sheet material having the shell structure, and found that the effect of improvement of its surface stiffness is insufficient as compared with the effect of improvement of its bending stiffness, and that even the bending stiffness of the sheet material is not sufficient for satisfying a requirement for further improvement depending on its application.